US 7689259 B2 Abstract A memory in a sensor is used to store multiple coefficients for a physiological parameter. In one embodiment, not only are the sensor's specific calibration coefficients stored in a memory in the sensor for the formula to determine oxygen saturation, but multiple sets of coefficients are stored. The multiple sets apply to different ranges of saturation values to provide a better fit to occur by breaking the R to SpO2 relationship up into different pieces, each described by a different function. The different functions can also be according to different formulas for determining oxygen saturation.
Claims(33) 1. An oximeter sensor comprising:
a light emitter for directing light at a patient;
a light detector mounted to receive light from the patient; and
a memory storing a first formula for determining oxygen saturation, a second formula for determining oxygen saturation, a first set of coefficients corresponding to a wavelength of the light emitter for use in the first formula, and a second set of coefficients corresponding to the wavelength of the light emitter for use in the second formula, wherein the first formula differs from the second formula.
2. The oximeter sensor of
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9. An oximeter sensor system comprising:
a light emitter for directing light at a patient;
a light detector mounted to receive light from the patient; and
a memory, mounted in a sensor or between the sensor and an oximeter monitor, the memory storing an indication of a breakpoint, first and second sets of coefficients corresponding to a same wavelength of the light emitter, and first and second formulas for determining oxygen saturation, the oximeter monitor selecting between the first and second sets of coefficients and the first and second formulas for determining oxygen saturation based at least in part on the breakpoint.
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16. An oximeter sensor comprising:
a light emitter for directing light at a patient;
a light detector mounted to receive light from the patient; and
a memory storing at least two different algorithms, and a plurality of alternate values of oxygen saturation or ratio-of-ratio values used in the at least two different algorithms to determine oxygen saturation, the plurality of values corresponding to the same mean wavelength of the same light emitter.
17. The sensor of
18. A pulse oximeter system comprising:
a pulse oximeter sensor comprising:
a light emitter for directing light at a patient;
a light detector mounted to receive light from the patient; and
a memory storing a first formula for determining oxygen saturation, a second formula for determining oxygen saturation, a first set of coefficients corresponding to a wavelength of the light emitter for use in the first formula, and a second set of coefficients corresponding to the wavelength of the light emitter for use in the second formula, wherein the first formula differs from the second formula; and
a pulse oximeter monitor configured to receive communications from the pulse oximeter sensor and configured to perform calculations using one or both of the first and second formulas to estimate oxygen saturation in blood of the patient.
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26. An oximeter system comprising:
an oximeter sensor, comprising:
a light emitter configured to direct light at a patient;
a light detector mounted to receive light from the patient; and
a sensor memory storing a first formula for determining oxygen saturation, a second formula for determining oxygen saturation, a first set of coefficients corresponding to a wavelength of the light emitter, and a second set of coefficients corresponding to the wavelength of the light emitter;
an oximeter monitor, comprising:
a calculation mechanism configured to determine a blood oxygen saturation in the patient, wherein the calculation mechanism selects and utilizes the first set of coefficients in the first formula for a first range of oxygen saturation values and selects and utilizes the second set of coefficients in the second formula for a second range of oxygen saturation values, wherein the first range differs from the second range.
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Description This application is a continuation of U.S. application Ser. No. 09/836,050, filed Apr. 16, 2001, now U.S. Pat. No. 6,801,797, which claims the benefit of U.S. Provisional Application No. 60/198,109, filed Apr. 17, 2000, which disclosures are incorporated herein by reference. The present invention relates to oximeter sensors having a memory. Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, and the rate of blood pulsations corresponding to a heart rate of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured. The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted or reflected light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation. Various methods have been proposed in the past for coding information in sensors, including pulse oximeter sensors, to convey useful information to a monitor. For example, an encoding mechanism is shown in Nellcor U.S. Pat. No. 4,700,708. This mechanism relates to an optical oximeter probe which uses a pair of light emitting diodes (LEDs) to direct light through blood-perfused tissue, with a detector picking up light which has not been absorbed by the tissue. The operation depends upon knowing the wavelength of the LEDs. Since the wavelength of LEDs can vary from device-to-device, a coding resistor is placed in the sensor with the value of the resistor corresponding to the actual wavelength of at least one of the LEDs. When the oximeter instrument is turned on, it first determines the value of the resistor and thus appropriate saturation calculation coefficients for the value of the wavelengths of the LEDs in the probe. Other coding mechanisms have also been proposed in U.S. Pat. Nos. 5,259,381; 4,942,877; 4,446,715; 3,790,910; 4,303,984; 4,621,643; 5,246,003; 3,720,177; 4,684,245; 5,645,059; 5,058,588; 4,858,615; and 4,942,877, the disclosures of which are all hereby incorporated by reference. The '877 patent in particular discloses storing a variety of data in a pulse oximetry sensor memory, including coefficients for a saturation equation for oximetry. Nellcor pulse oximeter sensors are encoded with a resistor (RCAL) value that corresponds to the wavelength(s) of the LED(s) within the emitter, such as described in U.S. Pat. No. 4,700,708. Nellcor pulse oximeter instruments read this resistor coding value and use it as a pointer to a look-up table that holds the proper set of coefficients for that sensor for calculating arterial oxygen saturation (SpO
Equation 2 can be further simplified to require only three constants (by, for example, dividing each constant by c A limitation of this method is that the proper calibration of the pulse oximetry sensor can be accomplished only if the relationship between the signal modulation ratio (R) to blood SaO A further limitation of this method is that the relationship between R and SaO A yet further limitation of this prior art method is that the functional relationship between the true arterial oxygen saturation and the measured signals may not fit a single function over the entire span of the measurement range. The present invention takes advantage of a memory in the sensor to provide enhanced performance. In one embodiment, not only are the sensor's specific calibration coefficients stored in a memory in the sensor for the formula to determine oxygen saturation, but multiple sets of coefficients are stored. The multiple sets apply to different ranges of saturation values to provide a better fit to occur by breaking the R to SpO2 relationship up into different pieces, each described by a different function. The different functions can also be according to different formulas for determining oxygen saturation. In another aspect of the invention, the sensor can store a variable breakpoint between the two functions used for oxygen saturation. The two functions could either be separate formulas or the same formula with different coefficients. This allows optimization to a value other than the 70% breakpoint of the prior art. In another aspect of the present invention, the sensor can store more than one breakpoint to create more than two functions describing the R to SpO2 relationship. In yet another aspect of the present invention, a spline function is used, breaking up the R to SpO2 relationship into an arbitrary number of regions. In one embodiment, the coefficients stored in the sensor memory correspond to a non-linear curve for low saturation values below 70% or some other breakpoint(s). Each of the methods described here improve the fit between the chosen mathematical function and the arterial oxygen saturation by breaking the relationship into subsets of the full measured range and determining optimum coefficients for each range. Spline-fitting, in this context, similarly breaks the full measurement range into subsets to efficiently describe the numerical relational between the underlying tissue parameter of interest and the actual signals being used to estimate its value. For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings. Sensor Reader/Monitor Light sources other than LEDs can be used. For example, lasers could be used, or a white light source could be used with appropriate wavelength filters either at the transmitting or receiving ends. Time Processing Unit (TPU) The signal received from input amplifier Sensor Memory Sensor In another embodiment of the invention, breakpoint β-Equation: In one embodiment, an enhanced form of the curvilinear function is used. Instead of using Eq.3 (linear) in the lower saturation region, Eq.2 (non-linear) is used for both the upper and lower saturation regions. The breakpoint that defines when to switch coefficients from an upper-region set to a lower-region set is defined by another coefficient. The breakpoint can be programmed either as a value of R, or as a value of SpO Curve fitting to multiple regions follows the same methodology as fitting to a single region. Simply put, the data is partitioned into separate regions and coefficients are determined for each region separately. Commercially available software programs are available, (for example, Mathcad, (Mathsoft, Inc., Cambridge, Mass.). The process can also be found in, for example, Data Reduction and Error Analysis for the Physical Sciences (Philip Beviyton, McGraw-Hill, New York 1969, Ch. 11—Least squares fit to an arbitrary function). Spline Fitting An alternate embodiment uses either spline (curve) fitting, or linear or higher order interpolation to a predefined set of SpO For instance, the values of R at specifically defined SpO - a) Only the bold values (e.g., a, b and c) would need to be stored with fixed, preselected spaced values of SpO
_{2 }(equally spaced or unequally spaced). Or, alternatively, preselected values of R. - b) An alternative approach would store within the sensor memory the SpO
_{2}(minimum) and SpO_{2}(maximum) values of the spline range, the number of knots that will be defined, and the sequence of defined values of R for those knots. - c) A further alternative approach could store both SpO
_{2 }and the associated R value for each knot. For each of these options, the instrument would use a spline-fitting algorithm, preferably a cubic spline, to determine the SpO_{2 }at the measured value of R according to the stored values (an alternative could be a linear or higher order interpolation algorithm).
There exists a trade-off in the number of knots defined and the amount of memory required to store them. Too few knots requires very little storage memory, but may not adequately describe the functional relationship; too many over-defines the curve and consumes more memory. The inventors have found that knots spaced 5%-10% apart give adequate results. Cubic Spline Calculation: The process for cubic spline interpolation is known to those skilled in the art. Intrinsic in using the spline method is that the value of R needs to be determined first before being translated to SpO The process of finding the coordinates of the knots in empirical data with a significant amount of noise may require an additional step. Commercially available basic curve fitting programs may be used (sigmaPlot, or TableCurve, or Mathematical for instance) to determine a best-fit functional approximation to the data. Alternately, one can perform a least-squares fit of an arbitrarily chosen analytical function and pick the values of R at the knot locations (SaO As will be understood by those of skill in the art, the present invention may be embodied in other specific embodiments without departing from the essential characteristics thereof. For example, any function can be used for the formulas for determining oxygen saturation, not just the ones described. For a limited sensor memory, the function representation may be compressed. Any representation of a function could be used. Calibration coefficients may be based on more or different characteristics than the sensor's LED wavelength(s). For example, other LED emitter characteristics or sensor design characteristics can be factors in the sensor's calibration coefficients. Additionally, the formula for calculating oxygen saturation may be a function of more than the ratio of ratios; for example, other input variables such as signal strength, light levels, and signals from multiple detectors could be used. This methodology for piece-wise fitting is not limited to oximetry. This method is useful when the relationship between the measured signal and reference value observed during calibration is not adequately described by a single function or set of coefficients over the whole measurement range. The relationship may be broken into subsets, and a piece-wise continuous set of functions may be used to describe the relationship. For example, other blood or tissue constituents could be calculated, such as carboxyhemoglobin, methemoglobin, bilirubin, glucose, lactate, etc. Accordingly, the foregoing description is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims. Patent Citations
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